The useful life of a perishable product is a function of its cumulative time-temperature exposurexe2x80x94a combination of the temperature to which a product is exposed and the duration of the exposure. Product degradation generally occurs more quickly at higher temperatures than at lower temperatures. Therefore, a perishable product will have a longer useful life if it is exposed to lower temperatures than if it is exposed to higher temperatures. Perishable products susceptible to degradation due to cumulative time-temperature exposure include, but are not limited to, food, food additives, chemicals, biological materials, drugs, cosmetics, photographic supplies and vaccines.
Many manufacturers mark their products with printed expiration dates in an attempt to provide an indication of when the useful life of a perishable product lapses. However, these dates are only estimates and may be unreliable because they are based on assumptions about the thermal history of the product that may not be true with respect to the particular unit of product in the package on which the expiration date appears. Specifically, manufacturers compute expiration dates by assuming that the product will be kept within a specified temperature range during the product""s useful life. However, the manufacturer cannot always predict or control the temperature exposure of a product through each step of the supply chain that delivers the product to the consumer. If the product is actually exposed to temperatures higher than those on which the expiration date is based, the perishable product may degrade or spoil before the printed expiration date. When this happens, the expiration date may mislead the consumer into believing that the perishable product is still usable when, in fact, its useful life has lapsed.
A time-temperature integrating indicator that provides a visually observable indication of cumulative time-temperature exposure is reported in U.S. Pat. No. 5,667,303, issued to Arens et al. (the ""303 patent). The device of the ""303 patent can monitor the cumulative time-temperature exposure of a single unit of a perishable product because the device can be attached to the product or product packaging. This provides the consumer with the benefit of being able to monitor the useful life of each individual unit from a common manufacturing lot of a perishable product. Because each unit may have a cumulative time-temperature exposures distinct from other units in the lot, the device of the ""303 patent can provide a more particularized and more accurate indication of the product""s useful life than a printed expiration date common to each unit in a manufacturing lot.
The device of the ""303 patent includes a first laminate wherein a substrate is coated with an opaque, porous matrix, a second laminate having a backing material coated with a viscous material (typically described and exemplified in the ""303 patent as a viscoelastic pressure sensitive adhesive), and an actuating means for activating the indicator. The viscous material and the porous matrix have the same, or approximately the same, index of refraction. Upon actuation, i.e., bringing the viscous material in contact with the porous matrix, the viscous material migrates into the porous matrix at a temperature-dependant rate. As microvoids of the opaque, porous matrix become filled with the viscous material, the porous matrix becomes transparent. As the matrix becomes transparent, a latent visually observable indication may become visible. Typically, the viscous material begins migrating into the porous matrix as soon as the two laminates are put in contact with one another.
The visually observable indication in the device of the ""303 patent is designed to occur when the cumulative time-temperature exposure of the device equals or exceeds the cumulative time-temperature exposure required to cause one or more changes in the perishable product sufficient to end the product""s useful life. Such changes include, but are not limited to, product deterioration, spoilage and loss of chemical or biological activity. The viscous material used in the indicator may be selected so that the runout time of the indicator, i.e., the time needed for the indicator to provide the visually observable indication, matches the time needed for the monitored change in the perishable product to occur.
The rate of degradation at a given temperature varies from product to product. Variation in the rate of degradation with respect to changes in temperature is also product-dependent; some products show a greater increase in the rate of spoilage for a given temperature increase than other products. Two measures for matching the temperature-dependent rates of change of the indicator and the perishable product are Q10 (an indication of how much faster a reaction occurs in response to a 10xc2x0 C. increase in temperature) and Ea (activation energy). As described in detail in the ""303 patent, one skilled in the art can use either the Q10 or Ea of a particular perishable product and known viscous materials in order to select a viscous material appropriate for use in an indicator for monitoring the particular perishable product.
One characteristic of the device reported in the ""303 patent is that the material employed as the viscous material begins migrating into the porous matrix as soon as the two laminates are put in contact with one another, i.e., upon completion of the actuation step. If the actuation step is omitted or delayed, the device may fail to indicate when the cumulative time-temperature exposure of the perishable product has exceeded a predetermined maximum. Conversely, if the actuation step occurs prematurely, the indicator may run out while the perishable product has useful life remaining, thereby creating waste. Thus, the device of the ""303 patent requires coordination between actuating the indicator and the initial exposure of the perishable product to potentially damaging temperatures in order to have the device provide an accurate indication of the product""s fitness for use. For some of the indicators reported in the ""303 patent, special or elaborate actuating means are required that could make the indicator more difficult to use, add to the cost of manufacturing, or both.
Furthermore, the indicators reported in the ""303 patent and elsewhere generally are designed to monitor the useful life of perishable products at cold temperatures over an extended period of time, e.g., several months or more. However, such indicators may not be suitable for monitoring a very short exposure (e.g., between about 1 and 24 hours) of the product to an abusive temperature (e.g., room temperature or higher). Consequently, if a perishable product is exposed for a short period of time to such abusive temperatures, its useable life may be shorter than the runout time of an indicator keyed to the product""s rate of change at lower temperatures. Thus, at abusive temperatures, the useful life of the product may lapse prior to the device indicating that the product is no longer fit for use.
In light of the foregoing, the present invention provides a time-temperature integrating indicator that is not necessarily activated (i.e., is not yet put into a xe2x80x9crunning statexe2x80x9d) when the two laminates are brought together. In the present invention, amorphous viscous material only begins to substantially migrate into the porous matrix (i.e., runout time is initiated) when the indicator is exposed to a predetermined temperature. Little or no migration of the amorphous viscous material occurs when the two laminates are brought together but maintained below a predetermined temperature. Further, the present invention provides a time-temperature integrating indicator that can monitor the cumulative time-temperature exposure of a perishable product for a short period of time at an abusive temperature.
In accordance with the foregoing, the present invention provides a time-temperature integrating indicator including a) a first substrate comprising a diffusely light-reflective porous matrix disposed thereon, and b) a second substrate comprising an amorphous material disposed thereon, the amorphous material including at least about 70% tackifier, by weight, wherein when the porous matrix and the amorphous material are in contact with one another at or above a predetermined temperature, the amorphous material migrates into the porous matrix at a rate that increases with increasing temperature, and wherein migration of the amorphous material into the porous matrix provides a readable indication of cumulative time-temperature exposure.
In another aspect, the present invention provides a time-temperature integrating indicator including a) a first substrate comprising a diffusely light-reflective porous matrix disposed thereon, b) a second substrate comprising an amorphous material disposed thereon, the amorphous material including at least about 30% tackifier, by weight, and no greater than about 33% adhesive base component, by weight, wherein when the porous matrix and the amorphous material are in contact with one another at or above a predetermined temperature, the amorphous material migrates into the porous matrix at a rate that increases with increasing temperature, and wherein migration of the amorphous material into the porous matrix provides a readable indication of cumulative time-temperature exposure.
In another aspect, the present invention provides a time-temperature integrating indicator including a) a first substrate comprising a diffusely light-reflective porous matrix disposed thereon, and b) a second substrate comprising an amorphous material disposed thereon, the amorphous material including no more than about 33%, by weight, adhesive base component and at least 30%, by weight, a viscous component having a ring-and-ball softening point from about 0xc2x0 C. to about 140xc2x0 C., wherein when the porous matrix and the amorphous material are in contact with one another at or above a predetermined temperature, the amorphous material migrates into the porous matrix at a rate that increases with increasing temperature, and wherein migration of the amorphous material into the porous matrix provides a readable indication of cumulative time-temperature exposure.
In another aspect, the present invention provides a time-temperature integrating indicator system for monitoring the deterioration of an object having separate deterioration profiles at about storage temperatures and at abusive temperatures. The system includes a plurality of indicators, each designed to provide a readable indication of cumulative time-temperature exposure of the object at a different predetermined temperature.
The present invention also provides a method of providing a readable indication of the cumulative time-temperature exposure of an object including the steps of a) providing a time-temperature integrating indicator including a first substrate having a diffusely light-reflective porous matrix disposed thereon and a second substrate comprising an amorphous material disposed thereon, b) potentiating the indicator by placing the amorphous material in contact with the porous matrix such that when the indicator is at or above the predetermined temperature, the amorphous material migrates into the porous matrix at a rate that increases with increasing temperature, thereby providing a readable indication of cumulative time-temperature exposure, and c) mounting the indicator on the object. The indicator may be potentiated prior to or after mounting the indicator on the object.
In another aspect, the present invention provides an indicator tape for use in a time-temperature indicator including a substrate and an amorphous material disposed upon the substrate.